An update -- After doing some additional investigation and cleaning of simple connections from starter button to starter, we got it to where it never failed on one battery but always failed on the other. So I decided to take the cables off the switch posts (one post at a time) and wire brush them. As I took off the cables from one of the posts of the switch in question (the one attached to the battery that won't start -- we did switch the battery cables and convinced ourselves both batteries were good), I noticed that the actual post was quite loose. I tightened up the post first, cleaned it, cleaned the cable connection, and then fastened them to the post. I also found the one of the ground post was slightly loose (not nearly as loose as the other). Anyway, with everything tight and cleaned, it starts on both batteries now. I'd love to say it fires right up, but you know diesels...it is not like starting a gas car engine. I'm a bit leery about whether this truly has resolved the issue. However, I've tried several times over the weekend to reproduce it and haven't been able to so ... time will tell.
"To reach the port of heaven, we must sail sometimes with the wind and sometimes against it - but we must sail, and not drift, nor lie at anchor.”
- Oliver Wendel Holmes